Process for drawing a polyamide yarn



March 28, 1%67 N. (5000 3 3Hfi91 PROCESS FOR DRAWING A POLYAMIDE YARN Filed Sept. 26, 1963 INVENTOR ALFRED NELSON GOOD ATTORNEY United States Patent 3,311,61 ROCESS F OR DRAWENG A PQLYAMTDE YARN Alired Nelson Good, Hixson, Tenn.,'assignor to E. 1. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sent. 26, 1963, Ser. No. 311,727 3 Claims. (Cl. 264-290) This invention relates to a process for drawing nylon yarn to produce products of increased stability, strength and toughness.

State of the art Polyamide filaments are well known to be highly useful for a wide range of textile products. Some of these products demand filaments having the highest tenacity possible. The maximum tenacity is customarily developed by drawing the filaments to the greatest extent consistent with good mechanical quality and freedom from breaks. Although the filaments find a ready acceptance in the market for tire cord and marine cordage, they have some drawbacks. For example, conventional drawing processes, especially at maximum draw ratios, produce a yarn which has a high shrinkage on heating or boilotf. The filaments also lack dimensional stability in that they tend to retract following the drawing operation. This retraction exerts tremendous crushing force upon the cores on which the filaments are wound; highcost, strong bobbins are required to withstand this force. In addition it has been observed that the highest tenacity yarn does not always result in the strongest rope or cordage.

To attain filaments of maximum tenacity it has been found that best results are obtained by drawing the yarn in two stages. In the first stage, the yarn is drawn over a snubbing pin (usually unheated), followed by a sec- 0nd stage of additional drawing in which the yarn is heated. A preferred process for this drawing is described in US. Patent 3,091,015 to I. Zimmerman.

The shrinkage inherent in nylon yarn produced by the above drawing processes may be reduced to a significant extent by annealing the yarn at high temperature at constant length, or by heating the yarn and allowing it to retract (relax) at the elevated temperature. The first of these alternatives usually produces a smaller reduction in shrinkage but gives a yarn of significantly higher strength and modulus. In contrast, heating the yarn while free to retract lowers modulus and tenacity while increasing break elongation, and maximum decrease in shrinkage is thereby obtained. Since maximum strength and modulus are especially important for tire cord and marine cordage use, relaxation treatments have been avoided for these yarns heretofore. Indeed, these properties are so important that high tenacity nylon yarn is always given an additional hot stretch after conversion to tire cord.

Objects It is an object of the present invention to provide a process for drawing a 'polyamide yarn to obtain a yarn of high tenacity and reduced retraction and shrink-age, suitable for production of marine type cordage of increased strength compared to filaments produced heretofore.

Statement of invention The process of this invention comprises the steps of forwarding a substantially undrawn polyamide yarn to a friction element to provide a first drawing by snubbing the said yarn about the said friction element and removing the said yarn from the said friction element at a linear speed of from about 2.2 to about 5.0 times the rate at which the said yarn is supplied to the said friction element, thereafter while further forwarding the said yarn, simultaneously (A) heating the said yarn through a fixed length of yarn path to a temperature of at least C. and less than 10 C. below the melting point of the said polyamicle (B) providing :a second drawing of the said yarn by removing the said yarn from the said length of yarn path at a linear speed at least about 1.] times the rate at which the said yarn is supplied to the said length of yarn path, thereafter maintaining the yarn at constant length while heating substantially at the temperature of the said length of yarn path for a period of from about 0.25 to about 0.60 second and finally cooling the said yarn while forwarding to windup at a linear rate of from about 0.985 to about 0.895 times the rate of the said yarn while maintained at constant length. Yarn produced in this manner is observed to have a toughness of at least 1.0 and to form rope and cordage of exceptional strength. By toughness as used herein is meant the area under the stress-strain chine. Its units are inch-grams/denier-inch.

The drawing The drawing is a schematic presentation of the steps of the present process. In the drawing driven roll 2 and associated separator roll 3 define a feeding means for yarn 1. Driven roll 5 and associated separator roll 6 form the components of the draw roll for the first drawing stage as well as the feed roll for the second stage. A snubbing pin 4 is provided as a frictional element in the first draw zone. The pin is conveniently made of abrasion resistant material such as aluminum oxide, sapphire, chromium plate or the like. The first draw zone is largely localized at pin 4. Yarn heating element 7 heats the yarn forwarded to the second draw zone. Element 7- is heated by internal electrical heaters powered by electrical leads 8 and 9. Element 7 is tubular with a hard chromium plate surface. The second stage of draw occurs at element 7 as the temperature of the yarn increasesto a temperature conducive to drawing. An-

nealing chest 10 is suppliedwith hot air through duct 11. Driven rolls 12 and 13 provide the tension for the second stage draw and maintain the yarn within chest 10 at a constant length. The hot chest is thermally insulated.

Driven roll 14 and associated separator roll 15 serve as :a tension let down system, and operate at a lower peripheral speed than rolls 12. and 13. A yarn guide 16 is associated with a conventional wind-up 17. There is a conventional yarn traversing mechanism not shown.

In operation, yarn 1 from a source not shown is forwarded about feed rolls 2, 3 in multiple wraps, and passes around snubbing pin 4. First stage draw roll 5 is operated at a surface speed higher than that of feed roll 2, whereby the yarn is drawn to a specified extent in the first stage while snubbed around pin 4. Yarn leaving roll system 5, 6 passes about the heated tube 7, whereby the yarn is raised to the required temperature and is subjected to drawing tension provided by rolls 12 and 13. An additional amount of drawing takes place in the sec- 0nd stage draw zone. The yarn is wrapped a number of times about rolls 12 and 13 whereby it is held at elevated temperature and constant length for the time specified. Since the yarn is hot as it leaves chest 10, it will retract spontaneously if given the opportunity to do so. This opportunity is provided by operating rolls 14, 15 at a lower peripheral speed than 12, 13, thus permitting the yarn to relax a predetermined amount. Since rolls 14, 15 121% unheated (except by contact with the hot yarn), the yarn is quenched by contact therewith, largely preventing further retraction in subsequent handling steps. Yarn, leaving rolls 14 and 15 is packaged on a conversion wind-up, such as the surface-driven no-twist windup indicated at 17.

The feed rolls 2, 3 may be replaced by a pinch roll system, or two driven rolls may be used.

Any snubbing pin or suitable snubbing device may be used instead of pin 4, as long the draw point is localized substantially on the snubbing device. For example, multiple pins may be employed about which the yarn passes in an S wrap. It is usually unnecessary to heat the snubbing element, since yarn friction normally produces enough heat to attain a satisfactory drawing temperature; pin temperatures of 20-85 C. are satisfactory, 50-85 C. being preferred.

It will be apparent that first stage draw roll combination 5, 6 may be as shown, or both may be driven. Alternatively, roll combination 5, 6 may be dispensed with, and a satisfactory distribution of draw ratio between the first and second stages obtained by adjusting the relative amount of snubbing on elements 4 and 7, and the temperature of element 7. As a guide in providing satisfactory distribution of draw ratio for such an arrangement, it should be noted that an increased snubbing on pipe 7 will increase the proportion of draw in the second stage and simultaneously reduce the draw occurring in the first stage at pin 4. An increase in temperature of pipe will also increase the draw ratio in the second stage, as it reduces the tension required for drawing in that stage. Alternatively, the use of a hot plate (reduced 4- the known devicesfor this purpose. A surface driven windup is highly suitable; the yarn may be traversed on to such a package by a cam, moving lever, grooved roll or the like. Alternatively, a conventional ring twister windup may be employed.

EXAMPLE Freshly spun 140 filament yarn of polyhexamethylene adipamide of 57 relative viscosity as measured in United States Patent No. 2,385,890 is processed following the procedure of the figure. The draw ratio in the first stage is 3.7x. In the second stage, the yarn makes a 1 /2 turn wrap on hot pipe 7. The pipe is 3 /2" in diameter and is heated to a temperature of 190 C. The total yarn path on the pipe is in. In hot chest 10, the yarn is wrapped 9 /2 times around rolls 12, 13. The supply air is at 250 C. to give a roll temperature of 215 C. The residence time in the enclosure is 0.3 sec. The yarn then passes to the tension let-down roll and the amount of relaxation listed in Table I is permitted. The tension in the relaxing zone .is measured and indicated in the table. The yarn is thereafter wound on a conventional zero twist windup. Yarn properties and processing conditions are listed in Table I, along with comparable results (sample A) obtained without heat on annealing chest 10. Under these conditions, yarn A relaxes only about 1%. The total draw ratio is 5.4x in stages 1 and 2, and the yarn denier (before relaxing) is 840.

TABLE I Sample A i B C l D E (control) l Annealing Temp, C 1 $0 195 215 195 195 Relaxation, percent ta. 1 1, 4, g 6 103. 5 Tension in Relax zone, gm 200 1, 250 2 700 450 110 Tenacity, g.p.d 9. (3 10.0 9. 9 9. 8 9. 4 Elongation, percent 16. L 15. 7 20. 3 19. O 22. 0 Initial modulus, g.p 52 53 43 39 Shrinkage, percent. 11. 3 l0. 7 7. 6 S. 2 5.0 Toughness (in.-gm./ .M. 0.98 1. (l0 1 14 1.02 1.13

1 Unheated. 2 Estimated.

snubbing) instead of the pipe will reduce the draw ratio in the second stage.

Heating element 7 is preferably an internally heated pipe as stated above; the pipe may be heated electrically or by hot oil, hot air, steam, or the like. It may also be replaced by a hot plate, a radiant tube, a bath containing hot liquid, or other suitable yarn heating device.

Annealing chest 10 is used to maintain the yarn and its associated drawing rolls 12, 13 at an elevated temperature; usually this will be about the same as pipe 7. It will often be necessary to supply the heated enclosure with hot air at a temperature higher than that which it is desired that the yarn will attain; for example, air at 240 C. may be suitably employed to anneal the yarn at 215 C. The number of wraps on rolls 12 and 13 and their peripheral speed will determine the length of time the yarn is maintained at the elevated temperature. According to the invention, this should be for a time of 0.25 to 0.60 second. In place of the heated oven, the yarn may be passed repeatedly over a hot plate-roll combination, or over internally heated rolls or the like.

The use of unheated relaxing rolls 14, 15 permits a controlled amount of yarn retraction, and also serves to cool the yarn to a temperature near that of the environment, thus stabilizing it for further processing. will sometimes be possible to eliminate rolls 14 and 15, when the windup is of the type which winds the yarn at a constant linear rate. This rate should be adjusted to permit the yarn to retract the required amount. In general, tension in this zone should be of the order of 0.2 to 1.5 grams/ denier.

A suitable windup for the yarn can be almost any of However, it.

The test is repeated, using hot rolls at C. For comparison purposes, a control sample F is prepared in which the same draw ratios are employed, but the chest 10 remains unheated, and the yarn is not relaxed. All yarns are twisted and plied to /2" diameter rope. Rope properties, corrected for denier differences, are listed in Table 2.

1 Unheatezl. 2 Estimated.

From the above it will be obvious that toughness of yarn is a characteristic determinative of rope strength. Furthermore, it will be obvious that toughness cannot be predicted from tenacity determinations alone. The present process provides yarns of unusual toughness, leading to cordage and rope of outstanding strength.

Processing conditions The preferred drawing conditions to be used in the first drawing zone are those disclosed by Zimmerman in US. 3,091,015 wherein polyamide yarns are drawn in two stages, the extent of drawing in the first stage being defined as a function of the birefringence B of the feed yarn. A wider range of first stage draw ratios may be employed for freshly spun than for prepackaged yarn. Due to yarn friction and consequent heating of the draw pin, drawing is normally carried out at 50 to 85 C. in the first stage.

Drawing in the second stage is preferably carried out above the force-to-draw transition temperature, as disclosed in the Zimmerman patent. This temperature is about 150 C. for polyhexamethylene adipamide. Higher temperatures are even more effective, as long as yarn damage and breakage are avoided. Temperatures of 190 to 230 C. are satisfactory. The draw ratio in the second stage should be at least 1.1x, and will normally be in the range of 1.2 to 1.9x. Total draw ratios of 2.5 to 5.5x and above may be used.

Following the second stage of drawing, the yarn is annealed at substantially constant length at a temperature equal to or above that employed in the second draw zone for from 0.25 to 0.6 second. The yarn may be cooled after drawing and reheated for the annealing step; it is preferred, however, to keep the yarn at drawing temperature so as to avoid the difficulties and waste of reheating.

Following the annealing step, the hot yarn is placed under a lower tension to allow it to retract a predetermined amount. The amount of retraction attainable will increase with the yarn temperature, and decrease with increasing yarn tension in the retraction zone.

In order to arrest the retraction of the hot yarn, it is preferably cooled by contact with an unheated roll which controls the amount of retraction permitted. Although this roll may attain a temperature of 80 C. from contact with hot yarn, this temperature (which is below the force-to-draw transition temperature) is sufiiciently low to cool the yarn. It is important that there be enough yarn wraps on this roll so that this yarn does not slip; this prevents windup tensions from affecting the uniformity and extent of retraction.

The process of this invention is specific to the treatment of polyamide yarns. By polyamide is meant those yarns formed from a polycarbonamide wherein the amide linkages are an integral part of the polymer chain. The invention has been illustrated by its application to filaments of polyhexamethylene adipamide, because of the great commercial importance of this polyamide. The process is also applicable to filaments of polycaproamide. Typical other polyamides are poly(aminoundecanoamide), poly(aminododecanoamide), polyhexamethylene sebacamide, poly(p-Xylylene-azelean-iide), poly(m-Xylylene adipamide), polyamide from .bis(p-arninocycloheXyD-methane and azelaic, sebacic and homologous aliphatic dicarboxylic acids. Other suitable polyamides are disclosed by Carothers in Us. Patents 2,071,250, 2,071,253 and 2,130,948. The invention is also applicable to copolymers and mixtures of these polyamides. It is within the purview of this invention to use polyamides which contain conventional delusterants, pigments, antioxidants and other additives as required.

Many equivalent modifications will be apparent to those skilled in the art from a reading of the above disclosure without a departure from the claimed concept.

What is claimed is:

1. A process comprising forwarding a substantially undrawn polyamide yarn to a friction element to provide a first drawing by snubbing the said yarn about the said friction element and removing the said yarn from the said friction element at a linear speed of from about 2.2 to about 5.0 times the rate at which the said yarn is supplied to the said friction element, thereafter, while further forwarding the said yarn, simultaneously (A) heating the said yarn through a fixed length of yarn path to a temperature of at least C. and less than 10 C. below the melting point of the said polyamide and ('13) providing a second drawing of the said yarn by removing the said yarn from the said length of yarn path at a linear speed at least about 1.1 times the rate at which the said yarn is supplied to the said length of yarn path, thereafter maintaining the yarn at constant length while heating substantially at the temperature of the said length of yarn path for a period of from about 0.25 to about 0.60 second and finally cooling the said yarn while forwarding to windup at a linear rate of from about-0.985 to about 0.895 times the rate of the said yarn while maintained at constant length.

2. The process of claim 1 wherein the said polyamide is polyhexamethylene adipamide.

3. The process of claim 1 wherein the said polyamide is polycaproamide.

References Cited by the Examiner UNITED STATES PATENTS 2,956,330 10/1960 Pitzl 264*290 3,124,632 4/1964 Larkin et a1. 264-290 ALEXANDER H. BRODMERKEL, Primary Examiner.

F. S. WHISENHUNT, D. J. ARNOLD,

Assistant Examiners. 

1. A PROCESS COMPRISING FORWARDING A SUBSTANTIALLY UNDRAWN POLYAMIDE YARN TO A FRICTION ELEMENT TO PROVIDE A FIRST DRAWING BY SNUBBING THE SAID YARN ABOUT THE SAID FRICTION ELEMENT AND REMOVING THE SAID YARN FROM THE SAID FRICTION ELEMENT AT A LINEAR SPEED OF FROM ABOUT 2.2 TO ABOUT 5.0 TIMES THE RATE AT WHICH THE SAID YARN IS SUPPLIED TO THE SAID FRICTION ELEMENT, THEREAFTER, WHILE FURTHER FORWARDING THE SAID YARN, SIMULTANEOUSLY (A) HEATING THE SAID YARN THROUGH A FIXED LENGTH OF YARN PATH TO A TEMPERATURE OF AT LEAST 130*C. AND LESS THAN 10* C. BELOW THE MELTING POINT OF THE SAID POLYAMIDE AND (B) PROVIDING A SECOND DRAWING OF THE SAID YARN BY REMOVING THE SAID YARN FROM THE SAID LENGTH OF YARN PATH AT A LINEAR SPEED AT LEAST ABOUT 1.1 TIMES THE RATE AT WHICH THE SAID YARN IS SUPPLIED TO THE LENGTH OF YARN PATH, THEREAFTER MAINTAINING THE YARN AT CONSTANT LENGHT WHILE HEATING SUBSTANTIALLY AT THE TEMPERATURE OF THE SAID LENGTH OF YARN PATH FOR A PERIOD OF FROM ABOUT 0.25 TO ABOUT 0.60 SECOND AND FINALLY COOLING THE SAID YARN WHILE FORWARDING TO WINDUP AT A LINEAR RATE OF FROM ABOUT 0.985 TO ABOUT 0.895 TIMES THE RATE OF THE SAID YARN WHILE MAINTAINED AT CONSTANT LENGTH. 